(a) Field of the Disclosure
This invention relates to motors of the kind that are driven by gas evaporated from a liquefied gas such as liquid carbon dioxide or liquid nitrogen.
(B) Description of the Prior Art
Motors which run on liquefied gases are known and have for instance been used to drive model aircraft (using sometimes a bottle of pressure-liquefied carbon dioxide).
A major difficulty which arises with such motors is the progressive fall in gas pressure which occurs as gas flows from the bottle or tank in order to drive the motor, and which limits the power of the motor to a rather low level. This fall in pressure is a consequence of the cooling of the gas as it attempts to evaporate from the liquid state in the supply bottle and to expand against ambient pressure during consumption by the motor. This cooling effect becomes worse as one attempts to increase the speed and power of the motor and can even cause formation of ice on the outside of the bottle. Furthermore the cooling of the gas causes its density to increase with the result that gas consumption is increased undesirably. A further disadvantage of existing motors powered by vaporised gas arises because the gas taken from the bottle for such existing motors is at or near the condition known as "saturation" with the consequence that, as soon as it is expanded in the motor, it inevitably condenses partly back into its liquid or even its solid state.
Apart from the possibility of damage to the motor such condensation also causes a large increase in the specific volume of the working fluid and this requires that the motor should have a high expansion ratio in order adequately to expand the working fluid and so extract its available energy, and this in turn leads to the need for an undesirably large motor, or to an undesirably low charge volume (which reduces motor power), or to the need for excessively high rpm in order to secure sufficient power from the motor. Our proposal for overcoming the problem of condensation of the gas in the motor is to superheat the gas before use in the motor, that is, to increase its temperature at sensibly constant pressure, or to reduce its pressure at sensibly constant temperature (or any combination of these two processes).
Although the value of superheating has sometimes been recognised in existing motors, the usual technique of achieving it has been by leading the gas from the supply bottle through fine-bore metal tubing before admission to the motor, this metal tubing usually being coiled and positioned so that ambient air flows over the tubing during operation. Existing motors are usually adapted to fly model planes and so have a propellor which blows air over the metal tubing. This technique gives a small but significant improvement in performance, though not overcoming the problem of condensation, diseconomy, power loss and possible motor damage at high power settings and, being reliant on the temperature of the ambient air to provide the superheating, is not very effective in cold weather when the extent of power loss can be severe. Moreover it does not give a significant expansion of the gas as it passes to the motor.
It is well known to heat by means of circulating warm fluids, parts of liquefied gas storage apparatus so as to prevent excessive frost from being deposited on such as apparatus during storage of the liquefied gas. Such heating systems are, for example, disclosed in U.S. Pat. Nos. 3,122,891 and 3,850,001.
It has been proposed in U.S. Pat. No. 3,466,196 to surround with a water jacket electronic equipment sealed in a shell before the shell is launched from the earth as part of an orbiting satellite or weather balloon. If the satellite or weather balloon reaches an altitude in the order of 30,000 meters it will be subjected to a temperature of -50.degree. to 60.degree. C. At such temperatures the water freezes thereby providing longterm protection of the equipment against extreme cold.